The measurement of acoustic waves that propagate in an elastic medium can provide useful information about the characteristics of the medium. For example, it is well known that mechanical disturbances can be used to establish acoustic waves in earth formations and the properties of these waves, also called seismic waves, can be measured to obtain important information about the formations through which the waves have propagated. In particular, parameters of compressional and shear waves, such as their velocity and polarization directions, can be indicators of formation characteristics that help in evaluation of the location and/or producibility of hydrocarbon resources. Reference can be made, for example, to my U.S. Pat. No. 4,809,239, assigned to the same assignee as the present application, which relates, inter alia, to a method for evaluating the velocity and direction of propagation of compressional and shear wave components propagating through earth formations, and to a technique for separating the individual waveforms of such components. Reference can also be made to U.S. Pat. No. 4,648,039, also assigned to the same assignee as the present application.
Detection of fractured zones and estimation of their properties, such as fracture orientation and fracture density, is of interest in exploration and production geophysics. The most prominent effect of aligned vertical (or near vertical) fractures on seismic waves is the splitting of shear waves. Theoretical predictions of shear-wave splitting has been confirmed by field observations over the last decade [e.g. S. Crampin, Evaluation Of Anisotropy By Shear-Wave Splitting, Geophysics 50, 142-152 (1985); D. F. Winterstein, Shear Waves In Exploration: A Perspective, 57th Ann. International Mtg., Soc. Expl. Geophys., Expanded Abstracts, 638-641 (1987)]. Shear wave splitting occurs when a shear wave separates into two phases with different velocities and different polarizations. Splitting is apparently caused by stresses, microcracks, or any other oriented inclusions in the formations. Once it is known that for fracture delineation one is looking for split-shear waves, the next question is how to find and extract information from split-shear waves in seismic data.
Multicomponent Vertical Seismic Profiles (VSPs) provide an excellent data set for the analysis of depth-dependent properties of fractured zones. FIG. 1 illustrates a simple shear VSP experiment for fracture delineation. Shear waves generated by a source 105 on the earth's surface are recorded in a borehole 102 by the horizontal (x,y) components of downhole geophones, represented at 111-115, as the waves propagate vertically across the horizontal layers. In regions containing oriented vertical fractures, shear waves split into two waves. The faster wave S.sub.1 is polarized along the fracture orientation and the slower wave S.sub.2 is polarized perpendicular to S.sub.1.
For detection and evaluation of split-shear waves several techniques have been proposed and applied to field data with varying degrees of success. S. Crampin (1985), cited above, uses the polarization direction of the initial (fast) shear wave to determine the fracture orientation and the onset of the second wave [indicated by the rapid turn-around in the hodogram--see D. F. Becker and A. I. Perelberg, Seismic Detection of Subsurface Fractures, 56th Ann. International Mtg. Soc. Expl. Geophys., Expanded Abstracts, 466-468 (1986)] to determine the delay between the split-shear waves. Hodograms, also known as polarization diagrams, illustrate mutually perpendicular particle displacements over a given time period. Hodograms represent point measurements in space, and are therefore very sensitive to local variations of the medium properties. However, in the presence of interfering waves, such as converted compressional (P) waves, multiples, and reflected waves, hodograms can be misleading and/or impossible to interpret.
In R. M. Alford, Shear Data In The Presence Of Azimuthal Anisotropy, 56th Ann. International Mtg., Soc. Expl. Geophys., Expanded Abstracts, 476-479 (1986), there is disclosed use of a source-receiver rotation procedure to separate the two shear waves in areas where there is only one dominant direction of azimuthal anisotropy (e.g., due to fractures). Once the data are rotated, the separated shear waves can be processed by conventional techniques to obtain the fast and slow shear interval velocities [D. H. Johnston, VSP Detection of Fracture-Induced Velocity Anisotropy, 56th Ann. International Mtg., Soc. Expl. Geophys., Expanded Abstracts, 464-466 (1986)].
Naville [Detection of Anisotropy Using Shear-Wave Splitting In VSP Surveys: Requirements and Applications, 56th Ann. International Mtg., Soc. Expl. Geophys., Expanded Abstracts 391-394 (1986)] proposes a technique using the cross correlations between two depth levels. It is assumed that the data consist of only two downgoing shear waves without any interference from reflections, conversions, etc.
L. Nicoletis et al. [Shear-Wave Splitting Measurements From Multishot VSP Data, 56th Ann. International Mtg. Soc. Expl. Geophys., Expanded Abstracts, 527-530 (1988)] propose a technique to obtain the transmission operator between two depth levels using two linearly independent source polarizations. The transmission operator, in turn, gives the fracture orientation, velocities and attenuations of two shear waves. Their technique also assumes only two shear waves without any interference. The transmission matrix is estimated at each frequency separately, whereas the fracture orientation and velocities (to a large degree) are independent of frequency.
It is among the objects of the present invention to provide improvement over the described types of prior art techniques.